1. Field of the Invention
The present invention relates to a permeation printed plastic body and a process and a system for conducting such a permeation printing, particularly to a styrene-based resin body.
2. Description of the Related Art
The printing of a plastic or synthetic resin article is carried out by hot stamp printing, silk screen printing, tampon printing, etc. For example, in devices such as a calculator, a pushphone, a television, a radio, a video player, a car stereo, and a computer, a number of push button type switches are arranged. Since the printed surfaces of these button switches are frequently pushed during operation of the device, the printing must be resistant to wear caused by friction and the letters, marks and figures must remain clear for a long time. However, prints obtained by methods such as hot stamp printing, silk screen printing, etc., of the prior art are less resistant to wear and are worn away by repeated push friction and finally completely worn away. Therefore, the development of process for providing a shaped plastic article having an excellent wear resistant print is desired.
Recently, permeation printing methods in which an ink image is formed or printed on a shaped surface of a plastic article and then permeated into the plastic article by heating have been proposed (for example, Japanese Unexamined Patent Publication (Kokai) Nos. 60-30344 and 59-199283). A modification of the permeation printing method has been proposed, in which a paper is placed on an ink image formed on a surface of a shaped article to absorb excess ink (Japanese Unexamined Patent Publication (Kokai) No. 59-199283). Another modification of a permeation printing method was proposed in which a dye migration-prevention layer of a water soluble resin is provided on an ink image formed on a surface of a shaped plastic article before heat migration of ink into the article (Japanese Unexamined Patent Publication (Kokai) No. 59-182779). However, these methods are primitive and not particularly suitable in practice for providing an excellent printing on a shaped article such as the top of a key button made of a polystylene-based resin.
Also, a transfer printing method was proposed in which a transfer sheet having patterns such as letters, marks or figures of a sublimating ink is brought into close contact with a shaped article of a polyester resin such as polybutyleneterephthalate (PBT), and then heated to print the letters, marks and figures on the shaped article by permeation (Japanese Unexamined Patent Publication (Kokai) Nos. 58-155957 and 59-199251). By selecting a suitable dye, a wear resistant printing is possible, but the formation of a shaped article of a polyester resin requires a high level injection molding technique and polyester resins are expensive compared with general purpose resins including a styrene-based resin.
On the other hand, if this transferring printing method for a polyester based resin is applied to a styrene-based resin, a high transfer temperature required due to a high molecular weight of a dye may often cause deformation of a styrene-based resin although such a high temperature does not cause deformation of a polyester resin. If transfer printing is conducted at a relatively low temperature, to prevent deformation of a polystyrene-based resin, an insufficient ink permeation depth is obtained and the wear resistance of the print is unsatisfactory. Further, the transfer printing method for a polyester-based resin does not provide a practically satisfactory print on a styrene-based resin.
A first object of the present invention is to provide a permeation printing method for a styrene-based resin, which can be conducted at a low temperature such that the styrene-based resin is not deformed and a sufficient ink permeation depth is attained.
Again referring to the transfer permeation printing method for a polyester-based resin, this method uses a transfer sheet in which dye patterns are printed in advance. Therefore, if contact of a transfer sheet with a resin is not uniform during a time that the dye sublimates and migrates from the transfer sheet to the resin, the transferred patterns may spread and be blurred by diffusion of the dye into air, so that the periphery of the patterns becomes unclear.
Moreover, the contents of the patterns which can be obtained on a printed resin are the same as the contents illustrated on a transfer sheet. As a result, when patterns printed on a key board are to be changed, the patterns of a transfer sheet must be changed, which disadvantageously necessitates the preparation of a changed transfer sheet.
Furthermore, in the transfer permeation method, the cost of the production of a key board becomes disadvantageously extremely expensive when the key board is to be manufactured in a small lot.
Furthermore, in the transfer permeation method, a transfer sheet is brought into contact with key tops and heating is effected from above the transfer sheet. If a transfer is conducted while keeping the key tops at a room temperature, key tops made of a resin are expanded and contracted by the heating from above the transfer sheet and slippage of the transferring sheet occurs as the temperature of the key tops increases, since the thermal expansion coefficients of the transfer sheet and the key tops are different. As a result, an insufficient coloration of the periphery, divergence of patterns, and blurred patterns appear. Therefore, a preliminary heating of key tops is necessary and an apparatus for this printing should comprise a preliminary heating unit and a heating unit for a transfer sheet from above thereof, which is a disadvantageously industrially complicated apparatus.
The second object of the present invention is to provide a simple system for a permeation printing of a shaped plastic article, which allows clear patterns to be stably printed at a high rate irrespective of the shape of the plastic article.